The Basics of Culvert and Inlet Design

[Pages:63]PDHonline Course C619 (8 PDH)

The Basics of Culvert and Inlet Design

Instructor: Jerry D. Morrow, PE

2020

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5272 Meadow Estates Drive Fairfax, VA 22030-6658 Phone: 703-988-0088

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PDHonline Course C619



The Basics of Culvert and Inlet Design

Introduction

The design of a culvert is influenced by cost, hydraulic efficiency, purpose, and the topography at the proposed culvert site. Thus physical data must be integrated with engineering and economic considerations. The information contained in this chapter should give the design engineer the ability to design culverts taking into account the factors that influence their design and selection. While computer programs are often used for design, the input data requires knowledge what effects the data has and what conditions must be evaluated. While most of the recommendations made herein are commonly accepted, they are the opinion of the writer and local standards will always prevail.

Definition

Culverts are structures used to convey surface runoff through embankments. Culverts are usually covered with embankment and composed of structural material around the entire perimeter, although some are supported on spread footings with the streambed serving as the bottom of the culvert. For economy and hydraulic efficiency, culverts should be designed to operate with the inlet submerged during flood flows, if conditions permit. Cross-drains are those culverts and pipes that are used to convey runoff from one side of a roadway to another.

Purpose

The primary purpose of a culvert is to convey surface water across or from the roadway right-of-way. In addition to the hydraulic function, a culvert must also support the embankment and roadway for traffic conveyance, and protect the traveling public and adjacent property owners from flood hazards to the extent practicable and in a reasonable and prudent manner.

Considerations

Primary considerations for the final selection of any drainage structure are that its design be based upon appropriate hydraulic principles, economy, and minimized effects on adjacent property by the resultant headwater depth and outlet velocity. In addition to sound hydraulic design, sound structural design, site design, and construction practices are necessary for a culvert to function properly. The allowable headwater elevation is that elevation above which damage may be caused to adjacent property and/or the roadway. It is this allowable headwater depth that is the primary basis for sizing a culvert.

To ensure safety during major flood events, access and egress routes to developed areas should be checked for the 100-year flood to determine if these streets will provide safe access for emergency vehicles and local residents.

Bridge or Culvert Selection

At many sites, either a bridge or a culvert will fulfill the structural and hydraulic requirements. The structural choice should be based on:

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PDHonline Course C619

1. Risk of property damage, 2. Construction and maintenance costs, 3. Traffic safety, 4. Environmental considerations, 5. Risk of failure, and 6. Aesthetic considerations.



Symbols, Definitions and Units ? TABLE 1

A

Area of cross section of flow sq. ft.

B

Barrel width ft.

Cd Overtopping discharge coefficient

D

Culvert diameter or barrel depth in. or ft.

d

Depth of flow ft.

dc Critical depth of flow ft.

du Uniform depth of flow ft.

g

Acceleration of gravity ft./sec.

H

Total energy loss ft.

He Entrance head loss ft.

Hf Friction head loss ft.

ho Height of hydraulic grade line above outlet invert ft.

HW Headwater depth above invert of culvert (depth from inlet invert to upstream total energy grade line) ft.

Ke Inlet loss coefficient

L

Length of culvert ft.

P

Empirical approximation of equivalent hydraulic grade line ft.

Q Rate of discharge cfs

S

Slope of culvert ft./ft.

TW Tailwater depth above invert of culvert ft.

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PDHonline Course C619

V

Mean velocity of flow ft./sec.

Vc Critical velocity ft./sec.



Concept Definitions

Critical Depth

Critical depth can best be illustrated as the depth at which water flows over a weir, this depth being attained automatically where no other backwater forces are involved. For a given discharge and cross- section geometry there is only one critical depth.

Uniform Flow

Uniform flow is flow in a prismatic channel of constant cross section having a constant discharge, velocity and depth of flow throughout the reach. This type of flow will exist in a culvert operating on a steep slope provided the culvert is sufficiently long.

Free Outlets

Free outlets are outlets whose tailwater is equal to or lower than critical depth. For culverts having free outlets, lowering of the tailwater has no effect on the discharge or the backwater profile upstream of the tailwater.

Submerged Outlets

Partially submerged outlets are outlets whose tailwater is higher than critical depth and lower than the height of the culvert. Submerged outlets are outlets having a tailwater elevation higher than the soffit of the culvert.

Submerged Inlets

Submerged inlets are those inlets having a headwater greater than about one and one-half times the diameter of the culvert.

Improved Inlets

Flared, improved, or tapered inlets indicate a special entrance condition which decreases the amount of energy needed to pass the flow through the inlet and thus increases the capacity of culverts at the inlet.

Soffit

Soffit refers to the inside top of the culvert. The soffit is also referred to as the crown of the culvert.

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PDHonline Course C619



Invert

Invert refers to the flowline of the culvert (inside bottom).

Steep and Mild Slope

A steep slope culvert operation is where the computed critical depth is greater than the computed uniform depth.

A mild slope culvert operation is where critical depth is less than uniform depth.

Culvert Design Steps

Following are the recommended steps in the design of a culvert in order to ensure that all design aspects are taken into account.

Step 1: Determine And Analyze Site Characteristics - Site characteristics include the generalized shape of the roadway embankment, bottom elevations and cross sections along the stream bed, the approximate length of the culvert, and the allowable headwater elevation. In determining the allowable headwater elevation, roadway elevations and the elevation of upstream property should be considered. The consequences of exceeding the allowable headwater elevation should be evaluated and kept in mind throughout the design process.

Culvert design is actually a trial-and-error procedure because the length of the barrel cannot be accurately determined until the size is known, and the size cannot be precisely determined until the length is known.

In most cases, however, a reasonable estimate of length will be accurate enough to determine the culvert size.

Step 2: Perform Hydrologic Analysis - Delineate the drainage area above the culvert site. Develop flow estimates for the design frequencies. The probable accuracy of the estimate should be kept in mind as the design proceeds.

Step 3: Perform Outlet Control Calculations And Select Culvert - These calculations are performed before inlet control calculations in order to select the smallest feasible barrel which can be used without the required headwater elevation in outlet control exceeding the allowable headwater elevation. The full flow outlet control performance curve for a given culvert (size, inlet edge, shape, material) defines its maximum performance. Therefore, the inlet improvements beyond the beveled edge or changes in inlet invert elevation will not reduce the required outlet control headwater elevation. This makes the outlet control performance curve an ideal limit for improved inlet design. The results of these calculations should be the outlet control performance curve. In addition to considering the allowable headwater elevation, the velocity of flow at the exit to the culvert should be checked to determine if downstream erosion problems will be created.

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Step 4: Perform Inlet Control Calculations For Conventional and Beveled Edge Culvert Inlets - Perform the inlet control calculations to develop the inlet control performance curve to determine if the culvert design selected will be on inlet or outlet control for the design and check flood frequencies. A drop may be incorporated upstream of the culvert to increase the flow through the culvert.

Step 5: Perform Throat Control Calculations For Side- Slope-Tapered Inlets - The same concepts are involved here as with conventional or beveled edge culvert design.

Step 6: Analyze The Effect of a Drop On Inlet Control Section Performance -The purpose of this step is to determine if having a drop before the inlet of the culvert would increase the capacity of the culvert and if a drop can be justified from a cost perspective and site characteristics.

Step 7: Design Side- and/or Slope-Tapered Inlet - Side- and slope-tapered inlets can be used to significantly increase the capacity of many culvert designs. Develop performance curves based on side- and/or slope tapered inlets and determine from a cost perspective and site characteristics if such a design would be justified.

Step 8: Complete File Documentation - Complete a documentation file for the final design selected.

Engineering Design Criteria

The design of a culvert should take into account many different engineering and technical aspects at the culvert site and adjacent areas. The following design criteria should be considered for all culvert designs as applicable.

Engineering aspects

1. Flood frequency 2. Velocity limitation 3. Buoyancy protection

Site criteria

1. Length and slope 2. Debris control

Design limitations

1. Headwater 2. Tailwater conditions 3. Storage

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Design options

1. Culvert inlets 2. Inlets with headwalls 3. Wingwalls and aprons 4. Improved inlets 5. Material selection 6. Culvert skews 7. Culvert sizes

Related designs

1. Weep holes 2. Outlet protection 3. Erosion and sediment control 4. Environmental considerations 5. Safety considerations

Loading requirements

Some culvert designs are relatively simple, involving a straight-forward determination of culvert size and length.

Other designs are more complex where structural, hydraulic, environmental, or other considerations must be evaluated and provided for in the final design. The design engineer must incorporate personal experience and judgment to determine which criteria must be evaluated and how to design the final culvert installation.

Expansion of the above criteria as it relates to culvert siting and design.

Flood Frequency

Culverts should be designed to convey at least the 50-year runoff event without overtopping the roadway.

The flow rate should be based on upstream full-build out land-use conditions from the tributary area.

Where roadside ditches convey the minor storm drainage in lieu of storm sewers, appurtenant culverts should be designed to convey the 10-year storm event, but in no case should be less than the minimum sizes specified by the regulatory authority.

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In addition, the 100-year frequency storm should be routed through all culverts to be sure structures are not flooded or increased damage does not occur to the roadway or adjacent property for this design event.

An economic analysis may justify a design to pass floods greater than those noted above where potential damage to adjacent property, to human life, or heavy financial loss due to flooding is significant.

Also, in compliance with the National Flood Insurance Program, it is necessary to consider the 100-year frequency flood at locations identified as being special flood hazard areas. This does not necessitate that the culvert be sized to pass the 100-year flood, provided the capacity of the culvert plus flow by-passing the culvert, is sufficient to accommodate the 100-year flood without raising the associated water surface elevation more than floodplain regulations or adjacent property elevations allow for that location. In addition, storm water management facilities cannot be installed which would result in a major lowering of the associated water surface elevation without a downstream evaluation. The design engineer should review the floodway regulations.

Velocity Limitations

Both minimum and maximum velocities should be considered when designing a culvert. The maximum velocity should not exceed culvert manufacturer recommendations. The maximum velocity should be consistent with channel stability requirements at the culvert outlet. As outlet velocities increase, the need for channel stabilization at the culvert outlet increases. If velocities exceed permissible velocities for the various types of nonstructural outlet material available, the installation of structural energy dissipaters is appropriate.

A minimum velocity of 3.0 ft./sec. when the culvert is flowing partially full is recommended to ensure a self-cleaning condition during partial depth flow. Energy dissipation may be required at the outlet of the culvert.

Buoyancy Protection

Headwalls, endwalls, slope paving or other means of anchoring to provide buoyancy protection should be considered for all flexible culverts. Buoyancy is more serious with steepness of the culvert slope, depth of the potential headwater (debris blockage may increase), flatness of the upstream fill slope, height of the fill, large culvert skews, or mitered ends.

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